1. Field of the Invention
The present invention relates to an apparatus for recording and reproducing digital signals on a magnetic tape and, more particularly, to an improvement thereof which can properly record and reproduce digital signals even when wow and flutter are present.
2. Description of the Prior Art
Recording and reproducing apparatus in digital form, known as a digital tape recorder, has been proposed in various systems and, one of which is disclosed in Japanese Patent Publication (unexamined) No. 92410/1984 issued May 28, 1984 (a counterpart U.S. Patent application is issued as U.S. Pat. No. 4,543,531 on Sep. 24, 1985, and a counterpart EPC application is published as A1 109837 on May 30, 1984).
According to this reference, an input data signal carrying digital information is sampled at a frequency which is M times higher than a channel bit rate. The sampled signals from two adjacent sampling points are used for calculating an interval from a zero cross-over point where the digital signal intersects a reference level to a sampling time. Then, using the calculated interval, a data detection pulse is generated for detecting the digital information in the data signal.
Also, according to this reference, a digital audio signal is pulse code modulated (PCM) into a k bit signal for being recorded on a magnetic tape having n tracks. The signals in each track are processed serially in a digital manner making it possible to share the hardware between the tracks, and at the same time, enabling the formation of circuits in the IC form.
The data signal is based on an eye pattern defined by the combination of sine and cosine waves having zero crossing points every after the channel bit rate T. Therefore, the obtained data signal, which is formed by connecting the selected upper half cycles and lower half cycles of the eye pattern, has zero crossing points at the phase "0". The digital information carried in the data signal is detected by data detection pulses which should be produced in a synchronized relationship with the phase "pi" of the data signal. It is to be noted that the phase "0" and phase "pi" are illustrated as an example. Thus, these phases can be opposite or any other phases can be used as long as they appear alternately and periodically. Accordingly, these phases are generally referred to as reference phases. To produce data detection pulses, the data signal is sampled twice between two subsequent "pi's" phases. However, since the sampling pulses are basically prepared separately from the data signal, the sampling pulses and the reproduced signal, although they have a related frequency as long as the tape runs at the rated speed, are not necessarily synchronized. In fact, they are not synchronized most of the time. Therefore, a suitable detecting means is necessary to detect the phase condition of the sampled data. Such a detecting means is disclosed in the above mentioned U.S. Pat. No. 4,543,531 or in EPC application publication No. A1 109837.
Also according to this apparatus, the data detection pulses for detecting the actual data signal are generated by using only one detecting means. For example, according to the above identified reference (U.S. Pat. No. 4,543,531), data detection signal .0.4 is generated by a single detection step using only one comparator 26 shown therein. However, when tape flutter takes place caused by the speed change of the tape, jitter and wow can occur. In such a case, different conditions must be considered to generate the data detection pulses, otherwise the data detection pulses will be skipped or undesirably added, resulting in a high rate of error.